AB08 N-Channel FET Characteristics

AB08 N-Channel FET Characteristics

Operating Manual Ver.1.1

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AB08 N-Channel

FET Characteristics Table of Contents

1. Introduction 4

2. Theory 6 3. Experiments 8

• Experiment 1 10 Study of the characteristics of JFET (Junction field effect transistor) in common source configuration and evaluation of:

4. Data Sheet 15 5. Warranty 17

6. List of Accessories 17

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Introduction AB08 is a compact, ready to use FET Characteristics experiment board. This is useful for students to plot different characteristics of an N channel Field Effect Transistor and to understand operation of an FET in different regions. It can be used as stand alone unit with external DC power supply or can be used with Scientech Analog Lab ST2612 which has built in DC power supply, AC power supply, function generator, modulation generator, continuity tester, toggle switches, and potentiometer. List of Boards :

Model Name AB01 Diode characteristics (Si, Zener, LED) AB02 Transistor characteristics (CB NPN) AB03 Transistor characteristics (CB PNP) AB04 Transistor characteristics (CE NPN) AB05 Transistor characteristics (CE PNP) AB06 Transistor characteristics (CC NPN) AB07 Transistor characteristics (CC PNP) AB09 Rectifier Circuits AB10 Wheatstone bridge AB11 Maxwell’s Bridge AB12 De Sauty’s Bridge AB13 Schering Bridge AB14 Darlington Pair AB15 Common Emitter Amplifier AB16 Common Collector Amplifier AB17 Common Base Amplifier AB18 RC-Coupled Amplifier AB19 Cascode Amplifier AB20 Direct Coupled Amplifier AB21 Class A Amplifier AB22 Class B Amplifier (push pull emitter follower) AB23 Class C Tuned Amplifier AB24 Transformer Coupled Amplifier AB25 Phase Locked Loop (FM Demodulator & Frequency Divider /

Multiplier) AB26 FET Amplifier AB27 Voltage Controlled Oscillator AB28 Multivibrator (Monostable / Astable) AB29 F-V and V-F Converter AB30 V-I and I-V Converter AB31 Zener Voltage Regulator AB32 Transistor Series Voltage Regulator AB33 Transistor Shunt Voltage Regulator AB35 DC Ammeter AB37 DC Ammeter (0-2mA) AB39 Instrumentation Amplifier

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AB41 Differential Amplifier (Transistorized) AB42 Operational Amplifier (Inverting / Non-inverting / Differentiator) AB43 Operational Amplifier (Adder/Scalar) AB44 Operational Amplifier (Integrator/ Differentiator) AB45 Schmitt Trigger and Comparator AB49 K Derived Filter AB51 Active filters (Low Pass and High Pass) AB52 Active Band Pass Filter AB54 Tschebyscheff Filter AB56 Fiber Optic Analog Link AB57 Owen’s Bridge AB58 Anderson’s Bridge AB59 Maxwell’s Inductance Bridge AB64 RC – Coupled Amplifier with Feedback AB66 Wien Bridge Oscillators AB67 Colpitt Oscillator AB68 Hartley Oscillator AB80 RLC Series and RLC Parallel Resonance AB82 Thevenin’s and Maximum Power Transfer Theorem AB83 Reciprocity and Superposition Theorem AB84 Tellegen’s Theorem AB85 Norton’s theorem AB88 Diode Clipper AB89 Diode Clampers AB90 Two port network parameter AB91 Optical Transducer (Photovoltaic cell) AB92 Optical Transducer (Photoconductive cell/LDR) AB93 Optical Transducer (Phototransistor) AB96 Temperature Transducer (RTD & IC335) AB97 Temperature Transducer (Thermocouple) AB101 DSB Modulator and Demodulator AB102 SSB Modulator and Demodulator AB106 FM Modulator and Demodulator

and many more…………

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Theory FET is a voltage controlled current device so its characteristics are the curves which represent relationship between different DC currents and voltages. These are helpful in studying different region of operation of a Field effect transistor when connected in a circuit. The two important characteristics of a Field Effect Transistor are: 1. Output /Drain characteristic.

2. Transfer characteristic.

Output / Drain Characteristics : It is the curve plotted between output drain current ID versus output drain to source voltage VDS for constant values of input Gate to source voltage VGS as shown in figure 1.

Figure 1 It can be subdivided into following four regions:

Ohmic region OA : This part of the characteristic is linear indicating that for low values of VDS, current varies directly with voltage following Ohm's Law. It means that JFET behaves like an ordinary resistor till point A (called knee) is reached.

Curve AB : In this region, ID increases at inverse square law rate upto point B which is called Pinch-off point. This progressive fall in the rate of increase of ID is caused by the square law increase in the depletion region at each gate up to point B where the two regions are closest without touching each other. The drain to source voltage VDS corresponding to point B is called pinch-off voltage VPO.

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Pinch-off region BC : It is also known as saturation region or 'amplifier' region. Here, JFET operates as a constant-current device because ID is relatively independent of VDS. It is due to the fact that as VDS increases channel resistance also increases proportionally thereby keeping ID practically constant at IDSS. Drain current in this region is given by Shockley's equation

It is the normal operating region of the JFET when used as an amplifier. ID = IDSS [1 – (VGS / VPO)2 ] = IDSS [ – (VGS / VGS (off))2 ]

Breakdown region : If VDS is increased beyond its value corresponding to point C (called avalanche breakdown voltage), JFET enters the breakdown region where ID increases to an extensive value. This happens because the reversed biased gate channel PN junction undergoes avalanche breakdown when small change in VDS produce very large change in ID.

JFET characteristics with External Bias : Figure 2 shows a family of ID versus VDS curves for different values of VGS. It is seen that as the negative gate bias voltage is increased: Pinch-off voltage VP is reached at a lower value of VDS than VGS = 0. Value of VDS for breakdown is decreased.

Figure 2

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Transfer Characteristic : It is the curve plotted between output drain current versus input Gate to source voltage for constant values of output drain to source voltage as shown in figure 3.

Figure 3

It is similar to the transconductance characteristics of a vacuum tube or a transistor. It shows that when VGS = 0, ID = IDSS and when ID = 0, VGS = VPO. The transfer characteristic approximately follows the equation

ID = IDSS [1 − (VGS / VPO)2 ] = IDSS [ 1 − (VGS / VGS(off))2 ]

The above equation can be written as VGS = VGS (off) [1 − (ID / IDSS)1/2 ] These characteristics can also be obtained from the drain/output characteristics by reading off VGS and IDSS values for different values of VDS.

The various parameters of a JFET can be obtained from its two characteristics. The main parameters of a JFET when connected in common source mode are

AC Drain Resistance, rd : It is the AC resistance between drain and source terminals when JFET is operating in the pinch-off region. It is given by rd = change in VDS at VGS constant or rd = VDS / ID | VGS

change in ID An alternative name is dynamic drain resistance. It is given by the slope of the drain characteristics in the pinch off region. It is sometimes written as rds emphasizing the fact that it is the resistance from drain to source. Since rd is usually the output resistance of a JFET, it may also be expressed as an output admittance yos. Obviously, yos = 1/rd. It has a very high value.

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Transconductance, gm : It is simply the slope of transfer characteristics

gm = change in ID at VDS constant or rd = ID / VGS | VDS

change in VGS

Its unit is siemens (S) /Mho. It is also called forward transconductance (gfs) or forward transadmittance Yfs. The transconductance measured at IDSS is written as gmo.

Mathematically

gm = gmo [1− (VGS / VP)] Amplification factor, µ : It is given by

µ = change in VDS at ID constant or µ = VDS / VGS | IDS

change in VGS

It can be proved from above that µ = gm × rd = gfs × rd

DC drain resistance, RDS : It is also called the static or ohmic resistance of the channel. It is given by

RDS = VDS / ID

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Experiment Objective : Study of the characteristics of JFET (Junction field effect transistor) in common source configuration and evaluation of: 1. AC drain resistance 2. Transconductance 3. Amplification factor 4. Drain Resistance Equipments Needed : 1. Analog board of AB08. 2. DC power supplies +12V,-5V from external source or ST2612 Analog Lab. 3. Digital Multimeter (3 numbers). 4. 2 mm patch cords.

Circuit diagram : Circuit used to plot different characteristics of transistor is shown in figure 4.

Figure 4

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Procedure : 1. Connect -5V and + 12V DC power supplies at there indicated position from

external source or ST2612 Analog Lab. 2. To plot Drain characteristics proceed as follows :

3. Rotate both the potentiometer P1 and P2 fully in counter clockwise direction.

4. Connect Ammeter between test point 3 and 4 to measure output drain current ID (mA).

5. Connect one voltmeter between test point 1 and ground to measure input voltage VGS other voltmeter between test point 2 and ground to measure output voltage VDS.

6. Switch ‘On’ the power supply. 7. Vary the potentiometer P2 so as to increase the value of output voltage VDS from

zero to 10V in step and measure the corresponding values of output drain current ID for different constant value of output voltage VDS in an observation table 1.

8. Rotate potentiometer P2 fully in Counter Clockwise direction.

9. Rotate potentiometer P1 and set the value of input gate to source voltage at some constant value (-1V, -2V, -3V……….)

10. Repeat the procedure from step 6 for different sets of input voltage VGS. 11. Plot a curve between output voltage VDS and output current ID at different

constant values of input gate to source voltage as shown in figure 2 using suitable scale with the help of observation table 1. This curve is the required output/Drain characteristic.

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Observation Table 1 :

Output Drain current ID (mA) at constant Value of input voltage S. no.

Output voltage

VDS (volt) VGS = 0V VGS = -1V VGS = -2V VGS = -3V

1. 0.0V

2. 1.0V

3. 2.0V

4. 3.0V

5. 4.0V

6. 5.0V

7. 6.0V

8. 7.0V

9. 8.0V

10. 9.0V

1. To plot Transfer characteristics proceed as follows :

2. Switch ‘Off’ the power supply. 3. Rotate both the potentiometer P1 and P2 fully in Counter Clockwise (counter

clockwise direction). 4. Connect voltmeter between test point 6 and ground to measure output voltage

VDS. 5. Connect one Ammeter between test point 3 and 4 to measure output current

ID(mA) 6. Vary potentiometer P2 and set a value of output voltage VDS at some constant

value (1 V, 2V, 3V..........) 7. Vary the potentiometer P1 so as to increase the value of input voltage VGS from

zero to maximum value in step and measure the corresponding values of output current ID in an observation table 2

8. Rotate potentiometer P1 fully in Counter Clockwise direction.

9. Repeat the procedure from step 5 for different sets of output voltage VDS.

10. Plot a curve between input voltage VGS and output current ID as shown in Figure 3 using suitable scale with the help of observation table 2. This curve is the required Transfer characteristic.

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Observation Table 2 :

Output Drain current ID (mA) at constant value of input voltage S. No.

Input voltage VGS (volt)

VDS = 1V VDS = 2V VDS = 3V VDS = 4V VDS = 5V

1. 0.0V

2. -0.5V

3. -1.0V

4. -1.5V

5. -2.0V

6. -2.5V

7. -3.0V

8. -3.5V

9. -4.0V

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Calculations : AC Drain Resistance, rd : It is the AC resistance between drain and source terminals when JFET is operating in the pinch-off region. To calculate AC drain resistance calculate the slope of the drain characteristics in the pinch off region obtained from Observation Table 1.

rd = change in VDS at VGS constant or rd = VDS / ID | VGS

change in ID It has a very high value.

Transconductance, gm : To calculate transconductance determine slope of the transfer characteristics obtained from Observation Table 2 gm = change in ID at VDS constant or rd = ID / VGS | VDS

change in VGS Its unit is siemens (S) / mho.

Amplification factor, µ : It is given by

µ = change in VDS at ID constant or µ = VDS / VGS | IDS

change in VGS

or µ = gm * rd

DC drain resistance, RDS : It is also called the static or ohmic resistance of the channel. It is given by

RDS = VDS /ID

Results : AC

Drain Resistance rd = _________ Transconductance, gm = _______

Amplification factor µ = _______ DC drain resistance, RDS _______________=

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Data Sheet

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Warranty 1. We guarantee the product against all manufacturing defects for 24 months from

the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.

2. The guarantee will become void, if

a) The product is not operated as per the instruction given in the operating manual.

b) The agreed payment terms and other conditions of sale are not followed.

c) The customer resells the instrument to another party. d) Any attempt is made to service and modify the instrument.

3. The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.

4. The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.

For any Technical Problem Please Contact us at [email protected]

List of Accessories

1. 2 mm Patch Cords (Red) ...................................................................... 2 Nos. 2. 2 mm Patch Cord (Black) ..................................................................... 2 Nos. 3. 2 mm Patch Cord (Blue) .........................................................................1 No.

4. e-Manual.................................................................................................1 No.

Updated 08-01-2009

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AB08 N-Channel FET Characteristics